1. Field of the Invention
The invention relates to the generation of AC voltage from either an AC or DC source. Such AC voltages may be used in induction and dielectric heating, ultrasonic cleaning and welding, audio, and battery powered AC mains replacement (“AC inverter”), or other uses. The AC output may also feed a load such as an AC to DC converter so that the final delivered output is DC.
2. Discussion of the Background
Previous output circuits have performed the function of generating an AC voltage. One implementation of such an output circuit, which uses two switching devices, is shown in FIG. 1. In this circuit 100, switching devices Q1 and Q2 are alternately turned on, neither being on at the same time. DC is blocked by C1 and AC is coupled to load RL, which could be a transformer, resonant circuit, piezo element or other device or circuit which requires large AC drive levels. Note that when either device Q1 or Q2 is turned on, the entire V1 voltage is applied to the device which is off.
Another popular implementation of an output circuit is shown in FIG. 2. Here, switching devices Q1 and Q4 are turned on at the same time, alternating with Q2 and Q3. Blocking capacitor C1 may or may not be used in a given implementation. Again, when a switching device is off, it can be seen that the full voltage V1 may appear across it. That is, for example, when Q1 is on and Q2 is off, Q2 will have essentially all of V1 across it, and must have a voltage rating so as to not fail in this condition.
Switching devices typically may be FET's, IGBT's, or other devices. Whatever technology is chosen for implementation, the devices used may have a maximum voltage specification. When the DC supply available is of high voltage, it may be difficult, expensive or impossible to select or obtain devices with an appropriate voltage specification to operate directly from this source and meet other system requirements. Sometimes, additional circuitry is required to reduce the voltage the switching devices will be exposed to, adding to cost and weight and reducing reliability.
There are also circumstances within integrated circuits where higher voltage devices may not be feasible. This could relate to linear circuits such as operational amplifiers and to switching circuits, such as drivers, or other circuitry.
Additionally, transient voltages or surges may be expected on some sources of applied DC, and if these voltages are directly applied across the switching devices then these devices would need a higher voltage rating for reliable operation.